1. Field of the Invention
The present invention relates to a three-dimensional optical memory for recording data on a recording medium in a multi-layer manner.
The present application is based on Japanese Patent Application No. Hei. 10-135307, which is incorporated herein by reference.
2. Description of the Related Art
Generally, single layer recording has been employed in the optical disk, the optical card, and the optical tape. Recently, a method of increasing recording surfaces in a multi-layer manner along the thickness (depth) direction to enhance a recording capacity to twofold, threefold, . . . has been proposed.
FIG. 5 is a view showing a schematic configuration of a writing once optical head 20 employed in the related single layer recording. In FIG. 5, an optical head 20 comprises a semiconductor laser 21, a collimator lens 22, a deflecting beam splitter 23, a xc2xcxcex plate 24, condensing lenses 25, 28, a condensing lens actuator 26, a half mirror 27, split detectors 29, 31, and signal amplifiers 35, 32, 33. The optical head 20 converges an output optical beam of the semiconductor laser 21 to irradiate it onto a surface of an optical disk 30. The optical head 20 outputs the optical beam of constant intensity at the time of reproducing, and outputs the optical beam whose intensity is modulated based on recording data at the time of recording. The optical head 20 is normally placed on a unidimensinal scanning stage and scans the optical beam two-dimensionally over the optical disk 30 which is rotated in the perpendicular direction to the scanning stage.
Meanwhile, in the multi-layer recording system in which the recording surfaces are increased in the thickness direction of the recording medium in a multi-layer manner, since a plurality of focused surfaces are formed along the optical axis direction, difference in a focusing distance in a focal length becomes very wide. Therefore, there is a problem in that the normal focusing format in the single layer according to the related art cannot be applied to the multi-layer recording system.
In order to eliminate the above-described problem, it is an object of the present invention to provide a three-dimensional optical memory which is able to facilitate focusing in a multi-layer recording.
In order to achieve the above object, according to the first aspect of the present invention, there is provided a three-dimensional optical memory which comprises a recording medium including a plurality of layers in a thickness direction thereof, a plurality of recording surfaces respectively formed on the layers, the recording surfaces each having data recording tracks, and sample servo marks, on which an optical beam is focused, provided respectively on the recording surfaces.
Since the sample servo marks are respectively provided on the recording surfaces of the layers, the recording surfaces can be detected precisely, and therefore, focusing in the multi-layer recording can be made easily.
According to the second aspect of the present invention, it is preferable that the sample servo marks each has at least two mark portions, the at least two mark portions are arranged in a zig-zag manner in the thickness direction of the recording medium while interposing one of the data recording tracks. As a result of this construction, identification of the sample servo marks can be facilitated. For example, when the number of the mark portions is set to two (2) and the optical beam emitted from the optical head is focused onto one of the mark portions, the optical beam is not focused onto the other one of the mark portions. Therefore, reflected beams from two mark portions have different intensities, respectively. The sample servo marks can be identified readily by detecting the reflected beams which have the different intensities, respectively.
According to the third aspect of the present invention, in view of variation of the focus of the optical beam, it is preferable that difference in positions of the at least two mark portions in the thickness direction of the recording medium is set larger than a focus depth (e.g., 2 to 3 xcexcm) of the optical beam which is emitted from an optical head.
According to the fourth aspect of the present invention, the three-dimensional optical memory further comprises a plurality of indication marks which are different from another depending respectively upon the recording surfaces, at least one of the indication marks being provided each of the sample servo marks on the data recording tracks. In accordance with the indication marks, respective recording surfaces can be easily identified. Incidentally, in FIG. 1, a recording surface of a first layer is indicated by xe2x80x9c0xe2x80x9d, a recording surface of a second layer is indicated by xe2x80x9c0, 1xe2x80x9d, and a recording surface of a third layer is indicated by xe2x80x9c1, 1xe2x80x9d.
According to the fifth aspect of the present invention, it is preferable that the indication marks include address codes respectively indicating the recording surfaces thereof.
Furthermore, according to the sixth aspect of the present invention, it is preferable that the sample servo marks and the indication marks are provided in plural numbers at a predetermined interval on the data recording tracks and lines which are perpendicular to the data recording tracks. In this case, the sample servo marks may be employed as samples in the astigmatism method.